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Wireless telephones - both mobile and cordless landlines - are possibly carcinogenic, according to an international report.

But next-generation cellphones are less harmful as they are more efficient and transmit at lower power levels.

A week-long meeting of 31 scientists in France for the World Health Organisation concluded radio-frequency magnetic fields from wireless phones were "possibly carcinogenic" to users.

This was based on evidence of an increased risk of glioma, a type of brain cancer.

The evidence, however, was considered limited for glioma and acoustic neuroma, "and inadequate to draw conclusions for other types of cancers".

Limited means a causal relationship is credible, "but chance, bias or confounding could not be ruled out with reasonable confidence".

The findings were released by the International Agency for Research on Cancer.

The head of the agency's working group on the issue, Dr Jonathan Samet, of the University of Southern California, said: "The conclusion means that there could be some risk, and therefore we need to keep a close watch for a link between cellphones and cancer risk."

Agency director Christopher Wild said until more research was done on longer-term heavy use of mobile phones, "it is important to take pragmatic measures to reduce exposure, such as hands-free devices or texting".

The agency said the scientists didn't quantify the possible risks, but it cited one study, up to 2004, which showed a 40 per cent increased risk of gliomas for the highest category of heavy users - an average of 30 minutes a day over a 10-year period.

Professor Alistair Woodward, the head of Auckland University's school of population health, said the "possibly carcinogenic" finding was significant.

"It does signal we can't put this issue to bed. We have to keep working on it ... [but] there's no sign of a lung cancer [and smoking] story."

The university had proposed studying people aged 10 to 24 for increased brain-cancer risk linked to cellphone use because past research was in adults, yet cellphones were now widely used by young people.

Martin Gledhill, senior science adviser at the National Radiation Laboratory, said there was uncertainty in the evidence relied on by the agency of a small increased brain-tumour risk for long-term cellphone users.

"The data on which this conclusion is based mostly arises from older technology phones. New technologies - 3G/XT/CDMA - produce much lower exposures than these phones."

Occupational health specialist Dr David Black, of Auckland University, said the agency had chosen the correct category. There was no confirmed evidence, nor even a seriously hypothesised mechanism by which cellphone use could cause cancer.